Please refer to FIG. 1, it shows the schematic circuit diagram of a first kind of low-power bandgap reference voltage circuits of the prior art. In which, the bandgap circuit includes three same kind of P-type MOSFETs 111, 112, 113, an operational-amplifier (op-amp) 12, two PNP type Bipolar Junction Transistors (BJTs) 131 and 132, and four resistors 14, 15, 161, and 162. Furthermore, the resistances of the resistors 161 and 162 are the same, and the cross measure of the p-n junction of the PNP transistor 132 is an integer factor multiplied by the cross measure of the p-n junction of the PNP transistor 131, and the integer factor is at least 2 such that the PNP transistor 132 can be formed by two PNP transistors each having the same cross measure of the p-n junction of the PNP transistor 131 with the same terminals of the two PNP transistors (the two emitters, the two bases, and the two collectors) coupled to each other respectively.
The two connecting nodes 101 and 102 formed at the two input terminal of the op-amp 12 are said to be virtually short-circuited such that the voltage values at the connecting nodes 101 and 102 are the same respectively. Thus, the difference between the base-emitter voltage of the PNP transistor 131 (VBE131) and the base-emitter voltage of the PNP transistor 132 (VBE132), ΔVBE132, equals to the voltage across the two terminals of the resistor 14 (as shown in FIG. 1), and which can be expressed as follows:ΔVBE132=VBE131−VBE132  (1)
Thus, the current flowed through the resistor 14 (having a resistance of R14) can be expressed as follows:iR14=ΔVBE132/R14  (2)
Since the voltage value at the connecting node 101 (VBE131) equals to the voltage value at the connecting node 102, the current flowed through resistor 162 (having a resistance of R162) can be expressed as follows:iR162=VBE131/R162  (3)
Since the current flowed through the drain of the P-type MOSFET 112 equals to the sum of the currents flowed through the resistors 14 and 162 respectively, and the three same kind of P-type MOSFETs 111, 112, 113 constitute a current mirror circuit, the current flowed through the resistor 15 (having a resistance R15) is the sum of the currents flowed through the resistors 14 and 162 respectively, and can be expressed as follows:iR15=iR14+iR162=ΔVBE132/R14+VBE131/R162  (4)
Thus, the bandgap reference voltage outputted from the connecting node 103 can be expressed as follows:Vref=iR15R15=R15(ΔVBE132/R14+VBE131/R162)=R15(IPTAT+IPTVBE)  (5)
When the circuit of FIG. 1 is compared to the traditional bandgap circuits, the item VBE131, which relates to the IPTVBE of the Vref equation (5), is multiplied by a factor, 1/R162, such that the output of the bandgap reference voltage Vref is relatively lower than the traditional bandgap reference voltages due to that two extra resistors 161 and 162 both having the same resistance are included. Through the properly choosing of the resistances of resistors 14, 15 and 162, the bandgap reference voltage outputted from the connecting node 103, Vref, would not be varied according to the absolute temperature since the ΔVBE132 and VBE131 are proportional to and inversely proportional to the absolute temperature respectively.
Two extra resistors, 161 and 162, both having the same resistance and the relatively high current value flowed through, are coupled to the terminals 102 and 103 respectively in the above-mentioned bandgap circuit for the purpose of achieving a relatively lower bandgap reference voltage. When the layouts of the ICs are under considerations, a relatively larger cross measure is needed for such a circuit, and which would become an unpractical drawback of this kind of bandgap circuits.
Please refer to FIG. 2, it shows the schematic circuit diagram of a second kind of low-power bandgap reference voltage circuits of the prior art. This bandgap circuit includes a current source 21, which offers a current proportional to the absolute temperature (IPTAT), a current source 22, which offers a current proportional to the base-emitter voltage (IPTVBE), PNP transistor 23, and the resistors 24 and 25 respectively.
Since the potential difference between the connecting node 201 and the connecting node 202 equals to the base-emitter voltage of the transistor 23 (VBE23), the current flowed through the resistor 24 (having a resistance of R24) can be expressed as follows:IPTVBE=VBE23/R24  (6)
The bandgap reference voltage outputted from the connecting node 203 can be expressed as follows:Vref=R15(IPTAT+IPTVBE)=R15(IPTAT+VBE23/R24)  (7)
Just like the aforementioned first kind of bandgap circuits, each of the second kind of bandgap circuits having an extra resistor 24 such that the item VBE23, which relates to the IPTVBE of the Vref equation (7), is multiplied by a factor 1/R24 such that the output of the bandgap reference voltage, Vref, is relatively lower than that of the traditional bandgap circuits. When compared with the first kind of bandgap circuits, only one resistor 24 having the relatively high current value flowed through for producing the IPTVBE is employed in each of the second kind of the bandgap circuits, but one more PNP type BJT 23 is employed though. Besides, the IPTAT and the IPTVBE are generated sequentially in each of this kind of circuits such that a relatively more complex configuration of the circuit is needed when it is compared with one of the first kind of bandgap circuits. But in the latter one, the IPTAT and the IPTVBE are generated simultaneously since a current mirror circuit is employed.
Please refer to FIG. 3, it shows the schematic circuit diagram of a third kind of low-power bandgap reference voltage circuits of the prior art. Each of this third kind of bandgap circuits includes a current source 31, which offers the IPTAT, a current source 32, which offers the IPTVBE, a resistor 33 coupled to a common ground, and a connecting node 30 providing a low-power reference voltage proportional to the sum of the IPTAT and the IPTVBE and coupled to the current sources 31 and 32 and the resistor 33. In which, the low-power reference voltage can be expressed as follows:Vref=R33(IPTAT+IPTVBE)  (8)
The basic theoretical configuration of this kind of bandgap circuits was first proposed by M. Gunawan, et. al, in the paper: “A Curvature-Corrected Low-Voltage Bandgap Reference”, IEEE J. of Solid-State Circuits, Vol. SC-28, No. 6, pp. 667–670, June 1993. The U.S. Pat. No. 6,366,071 B1 (H. C. Yu) was built on the above-mentioned basic configuration (as shown in FIGS. 3 and 4 of the '071 Patent). But, the detailed configuration of the bandgap circuits disclosed in the '071 Patent is relatively complex having ten MOSFETs, three BJTs, and two resistors (as shown in FIG. 5 of the '071 Patent). To build up a new kind of bandgap circuits each having a much simpler configuration and the same level of efficiency according to the aforementioned basic theoretical configuration would be the next challenge.
Keeping the drawbacks of the prior arts in mind, and employing experiments and research full-heartily and persistently, the applicant finally conceived the low-power bandgap circuits having relatively less components.